ui/spice-input: Remove superfluous forward declaration

We only need to forward-declare kbd_push_key() and kbd_get_leds()
which are used in kbd_interface, not kbd_leds(). Remove this
superfluous forward declaration.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200909171145.350360-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>